The drive train of a motor vehicle is configured to transfer power from the vehicle's motor to the vehicle's wheels. The drive train typically includes a transmission that includes a gearing assembly that provides the vehicle with a set number of available speeds (also referred to as gear ratios). For example, a six speed automatic transmission has first gear, second gear, third gear, fourth gear, fifth gear, sixth gear, and reverse. The ratio between the rotational speed of the drive shaft (output shaft of the motor) and the output shaft of the transmission is different for each gear/speed.
For example, the gear ratio for 1st gear could be three to one (3:1) and the ratio for sixth gear could be one-half to one (0.5:1). For the transmission in the above example, when the vehicle is in first gear the output shaft of the engine (i.e., input shaft of the transmission) rotates three times for each rotation of the output shaft of the transmission. This ratio (“low” gear) is used when the vehicle is starting from a stop and for relatively slow speeds. On the other hand, when the transmission in the above example is in sixth gear, the output shaft of the engine rotates one-half of a revolution for each rotation of the output shaft of the transmission. This ratio (“high” gear) is used when the vehicle is traveling at high speeds. The drive train can also include differentials which transfer torque from the transmission to the axles coupled to the wheels of the vehicle.
The ratio between the number of rotations of the engine and the number of rotations of the axle shaft (wheel rotations) is referred to as the overall gear ratio. This ratio is the product of the transmission ratios described above as well as the final drive ratio which is defined by the gearing in the differentials (e.g., number of teeth on the final drive pinion gear and the final drive ring gear). For example, if the final drive ratio is three to one (3:1), the overall ratio for first gear in accordance with the example above would be nine to one (9:1) which is the product of three (first gear ratio) times three (the final drive ratio). This ratio would be the launch ratio since it is used to launch the vehicle from a standstill.
To maximize the performance of the engine at full-throttle (e.g., horse power, torque, fuel efficiency) it is desirable to operate the engine in its particular target speed range (e.g., the engine speed (rotations per minute (RPM)) generally associated with the RPM near and between the peak torque to peak horsepower commonly known as the “power band”). Automatic transmissions are configured to automatically shift the transmission to keep the engine speed in this power band target range during full-throttle operation. However, when at “part-throttle” or “light-throttle” conditions that often exist when the vehicle is cruising, or accelerating at a less-than-maximum rate, or otherwise in a condition that requires less-than-full-throttle, the desired target engine speed is likely lower than that of full-throttle to maximize fuel efficiency and driver comfort. The shift timing and sequence is based on a number of factors including the speed of the vehicle and performance desired (e.g., maximum acceleration, maximum fuel efficiency, etc.), and is usually controlled by one or more system inputs (e.g., throttle position, engine and vehicle speed, oil temperature, etc). For example, to maximize fuel efficiency while cruising at a fairly constant speed, it is generally desirable to maintain the engine speed at a relatively low speed that is still high enough to have sufficient power to maintain the desired driving speed.
All other factors being equal, transmissions with more available gear ratios and wider overall ratio spreads enable the vehicle to be run more effectively since it is more likely that the optimum target engine speed can be maintained. For example, a particular engine when paired with a nine speed transmission will typically be more fuel efficient than the same engine paired with a four speed transmission. A vehicle with more available speeds can also be smoother riding since the engine speed can be kept closer to constant while the wheel speed varies, resulting in a relatively consistent feel. Practical design constraints (e.g., limits on the space available for additional gearing, undesirability of adding weight to the assemblies, undesirability of adding complexity in shift controls, energy loss due to additional friction in the drive train, etc.) limit the number of speeds of commercially available transmissions. There is a need in the art to provide vehicles with more gear ratios while addressing these and other design constraints.